1. Field of the Invention
The present invention relates to heating systems. More specifically, the invention is a system and method for increasing heating efficiency in heat pumps.
2. Description of the Related Art
Heat pump systems for residential and commercial heating and air conditioning applications are well known among heating, ventilating, and air conditioning (HVAC) systems, and are popular for their relative energy efficiency in a broad range of heating operations.
A heat pump system typically uses an outdoor heat-exchanging coil to extract heat from outdoor air. Refrigerant flowing through the coil is heated, and pumped through an indoor heat-exchanging coil. An air-circulating blower moves indoor air through ductwork and over the indoor heat-exchanging coil, warming the air.
It can be readily recognized that, as outdoor temperatures drop, it becomes increasingly difficult for the heat pump to extract heat from colder outdoor air. As outdoor air becomes colder, the refrigerant temperature delivered to the indoor heat-exchanging coil drops, reducing the amount of heat transferred to the indoor air. Especially after the unit first starts in response to a thermostat command, and before the refrigerant and indoor coil have had a chance to become fully heated, heat pump units often circulate uncomfortably cool air into interior living spaces, when outdoor temperature are low, in a phenomenon known as a “cold blow”.
A cold blow may additionally be experienced as a heat pump system undergoes a defrost cycle. During operation, the heat pump's outdoor heat-exchanging coil becomes chilled, colder still than the outside air, as heat is transferred to the refrigerant. Thus, it is common, under certain operating conditions, for frost to form on the outdoor heat-exchanging coil, reducing efficiency and ultimately preventing the heat pump's operation if frost buildup is excessive. Heat pump systems often go into a reverse cycle, functioning for a brief period in an air conditioning mode whereby the outdoor heat-exchanging coil is warmed to eliminate frost. During this reverse cycle, the indoor heat-exchanging coil is cooled, rather than heated, contributing to a cold blow.
Supplemental electric resistance heating elements are often used to heat the indoor air during defrost cycling, and to provide additional heating during periods when low outdoor air temperatures prevent the heat pump warming indoor air to comfortable levels. The use of supplemental electric resistance heating elements, however, increases energy requirements for the system, resulting in a decrease in heating efficiency.
Various systems and methods have been employed to improve efficiency of heat pump operation, and to reduce cold blow effects during defrost cycling and during periods when low outdoor air temperatures hinder operation of the heat pump.
U.S. Pat. No. 6,131,402, issued on Oct. 17, 2000 to E. Mills, Jr. et al., discloses an apparatus and method of operating a heat pump to improve heating supply air temperature. A temperature sensor, placed proximate to the outside coil, which functions as the evaporator during heating operations, monitors the outside ambient air temperature. Blower speed is set according to the outdoor ambient air temperature. A supply air sensor may be included to sense the air flow rate or air temperature at the exit of a condenser duct, providing a closed loop determination of motor speed based on the sensed supply air characteristic and target air flow.
U.S. Pat. No. 5,202,951, issued on Apr. 13, 1993 to E. Doyle, discloses a mass flow rate control system and method for controlling a variable speed motor to drive a blower to maintain a desired air flow under varying resistances of a heating system. The system varies the blower speed to maintain a constant mass flow rate under variations in air flow resistance in the heating system.
U.S. Pat. No. 5,492,273, issued on Feb. 20, 1996 to R. Shah, discloses an HVAC system having a variable speed indoor blower motor. A motor speed controller provides for two, or three, defined airflow rates. Rather than continuously varying the motor speed according to indoor coil temperature and supply duct temperatures to match BTU output to airflow, the controller sets the motor speed for one of the defined air flow rates according to an operational state of the system.
U.S. Pat. No. 4,860,552, issued on Aug. 29, 1989 to T. Beckey, discloses a heat pump fan control that provides a variable time delay between startup of the compressor and startup of the blower fan. The delay time is determined by the outdoor air temperature, and increases as the outdoor temperature decreases.
U.S. Pat. No. 4,627,484, issued on Dec. 9, 1986 to J. Harshbarger, Jr. et al., discloses a heat pump control system that monitors defrost cycling of the heat pump, and shuts down the heat pump if the defrost cycle continues for an excessive length of time or if outside ambient temperature is excessively low. The heat pump is thus disabled when weather conditions do not favor efficient heat pump operation.
U.S. Pat. No. 4,364,237, issued on Dec. 21, 1982 to K. Cooper et al., discloses a heat pump system in which the compressor speed is varied in response to load conditions. The indoor fan speed is varied according to the compressor speed And the outdoor air temperature.
U.S. Pat. No. 4,324,288, issued on Apr. 13, 1982 to P. Karns, discloses a level supply air temperature heat pump system and method. The air temperature of air discharged from the indoor heat exchanger of a heat pump system is measured and used to control the indoor fan speed to maintain the supply air temperature at a relatively constant level.
U.S. Pat. No. 3,339,628, issued on Sep. 5, 1967 to W. Sones et al., discloses an electrically controlled heating system wherein a fan motor is operated at variable speeds. The system uses temperature sensors located variously within the living spaces to vary the fan motor speed during both heating and cooling operations, but does not measure the available heat output during heating operations to set the fan motor speed for optimal heat transfer.
U.S. Pat. No. 4,408,713, issued on Oct. 11, 1983 to T. Iijima et al., discloses a control for automobile air conditioning systems wherein the airflow rate is controlled in relation to a sensed ambient air temperature and the temperature of a heat source for the system. The airflow rate is increased gradually over a time interval after the system is activated. The rate of increase is determined by the temperature of the system heat source, but the airflow rate does not track changes in the temperature of the heat source.
None of the above inventions and patents, taken either singly or in combination, is seen to describe the instant invention as claimed. Thus a method of increasing efficiency of heat pumps solving the aforementioned problems is desired.